Put the chicken and the chicken stock into a largeish saucepan and bring to a boil. At this stage, add the salt, pepper, ground coriander, ground cumin, saffron, tomatoes, half the onion, cinnamon quill, ginger, garlic and chilli powder. Simmer for 30 minutes. Remove the pan from the heat and set aside.
In a small frypan, heat the peanut oil and brown the remaining onion with the crumbled curry leaves (this is to 'temper' the mollagathanni: tempering is a Sri Lankan cooking technique). When the onions are browned, add the coconut milk. Then pour the whole shebang into the chicken mixture. The chicken is now tempered.
Add the lemon juice to the mollagathanni. Bring it to a boil, then reduce the heat and simmer the soup for 15 minutes.
Serve hot as a stand-alone soup or with basmati rice and curries on the side.